1. Technical Field of the Invention
The present invention relates generally to light engines for projection display devices, and more particularly to an improvement that takes advantage of color switching and/or both polarizations of light generated by their light sources.
2. Background Art
FIG. 1 illustrates a light engine kernel according to the prior art. It includes four polarization beam splitters (PBS""s) 101-104 and three spatial light modulators (SLM""s), as well as a number of polarizers and filters as needed. A PBS is typically constructed as a cube of glass made from two prisms with an optical coating on their mating surfaces which reflects incoming xe2x80x9cSxe2x80x9d polarized light and passes incoming xe2x80x9cPxe2x80x9d polarized light. An SLM is a multi-pixel opto-electronic device that modulates the intensity of light that is imaged to its pixels, by reflecting (or in some embodiments by transmitting) controllable amounts of light independently at each pixel. One embodiment of an SLM is constructed as a liquid crystal on silicon (LCOS) microdisplay. Other embodiments may be constructed as liquid crystal displays (LCD""s), digital mirror devices (DMD""s), grating light valves (GLV""s), or other suitable technologies.
The light engine kernel 100 of FIG. 1 receives an incoming light beam Lr which has been pre-polarized and pre-filtered into color bands such as those suitable for use in a red-green-blue (RGB) color space. Let the notation xe2x80x9cXyxe2x80x9d denote color (X) and polarization (y). The pre-polarization serves to reflect the s polarization of the incoming light (Ls) back to the light source (or for a PBS polarizer redirects it into an absorber). The GM filter serves to rotate the polarization state of the light in the green band Gp to the s polarization state Gs, while leaving the polarization of the light in the blue and red bands unchanged Bp, Rp. The incoming light arrives at the first PBS 101 which reflects the Gs component upward and passes the Bp and Rp components of the incoming light. The green Gs light arrives at the second PBS 102, which reflects the Gs light onto the Green SLM which modulates the Gs light to insert the green component of the image to be displayed. After passing once through the quarter-wave plate in each direction, the Gs light will have been repolarized to Gp, which the second PBS 102 will pass to the fourth PBS 104.
The Bp and Rp components are passed from the first PBS to the third PBS 103, which passes the Bp light through to a Blue SLM and reflects the Rp light to a Red SLM. After being repolarized by the quarter-wave plates, the blue light is reflected and red light is passed to the fourth PBS.
The fourth PBS 104 recombines the green beam with the red/blue beam, to form an output beam.
One disadvantage of this system is that it requires the discarding of one polarization of the input beam. Typically, half of the light source""s output is discarded. This reduces the intensity of the output beam and thus the brightness of the image on the screen.